linux-kernel - Re: [PATCH] pagemap: fix wrong KPF

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Date:	Wed, 26 Sep 2012 15:38:41 +0800
From:	Fengguang Wu <fengguang.wu@...el.com>
To:	Naoya Horiguchi <n-horiguchi@...jp.nec.com>
Cc:	David Rientjes <rientjes@...gle.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Andi Kleen <andi.kleen@...el.com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] pagemap: fix wrong KPF_THP on slab pages

On Wed, Sep 26, 2012 at 02:06:08AM -0400, Naoya Horiguchi wrote:
> On Wed, Sep 26, 2012 at 12:02:34AM -0400, Naoya Horiguchi wrote:
> ...
> > > > +	 * page is a thp, not a non-huge compound page.
> > > > +	 */
> > > > +	else if (PageTransCompound(page) && !PageSlab(page))
> > > >  		u |= 1 << KPF_THP;
> > > 
> > > Good catch!
> > > 
> > > Will this report THP for the various drivers that do __GFP_COMP
> > > page allocations?
> > 
> > I'm afraid it will. I think of checking PageLRU as an alternative,
> > but it needs compound_head() to report tail pages correctly.
> > In this context, pages are not pinned or locked, so it's unsafe to
> > use compound_head() because it can return a dangling pointer.
> > Maybe it's a thp's/hugetlbfs's (not kpageflags specific) problem,
> > so going forward with compound_head() expecting that it will be
> > fixed in the future work can be an option.
> 
> It seems that compound_trans_head() solves this problem, so I'll
> simply use it.

Naoya, in fact I didn't quite catch your concerns. Why not just test

        PageTransCompound(page) && PageLRU(page)

The whole page flag report thing is inherently racy and it's fine to
report wrong values due to races. The "__GFP_COMP reported as THP",
however, should be avoided because it will make consistent wrong
reporting of page flags.

Thanks,
Fengguang
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